This invention relates to a protractor for measuring the blade angle of a propeller, fan and the like and particularly to the incorporation of an electronic shaft angle encoder and a complementary digital readout coupled to a movable carriage for positioning the sine bar in relationship to the propeller, blade station and supporting table.
The prior art protractor is exemplified in U.S. Pat. No. 2,481,062 granted to O. L. Anderson on Sept. 6, 1949 and assigned to the same assignee and typifies the type heretofore used for measuring blade angle. Because this type of protractor is positioned, held in place and set solely by the operator, and the reading of the scale is no better than the accuracy of the reader, such a device while adequate is not sufficiently accurate for certain aircraft applications. Not only is it difficult to obtain repeatability when a single operator makes a measurement, the inaccuracies are generally compounded when more than one operator does the reading.
Thus, in applications where it is desirable to obtain an accuracy of say 0.01 of a degree of the blade angles amongst the blades in a propeller, regardless of the number of blades, the heretofore measurement devices are not adequate. We have found that we can obviate the problems of the heretofore known protractors by leveling and fixing the protractor base to the table, allowing the protractor to move perpendicular and parallel to the table top surface while restraining it from tilting or cocking in any direction and employing a shaft angle encoder mounted through a flexible bellows that is electronically connected to a complementary digital readout so that the blade angle is seen in digits in the window of the viewer and hence eliminates the necessity of the viewer from reading a scale, as the case was heretofore.